Combining different analytical approaches to identify odor formation mechanisms in polyethylene and polypropylene

Abstract

In a previous study, we identified carbonyls as highly odor-active compounds in both unprocessed and processed polypropylene (PP) with higher intensities after processing, indicating a temperature-driven forming mechanism. In the presented work, we studied whether (a) these carbonyls are the major odor drivers to the overall odor of polyolefins, (b) their formation is taking place already at moderate temperatures well below the typical processing temperatures, (c) conventional antioxidants in polyolefins can prevent or reduce their formation, and (d) whether reducing the amount of oxygen present can decrease the overall odor. One polyethylene (PE) and one PP were selected, and both stabilized and unstabilized polymer powder samples were exposed to conditions differing in oxygen concentration and aging time. The changes in the volatile fraction as well as the formation of odor-active compounds were monitored using a multidisciplinary approach by combining analytical methods based on gas chromatography (GC), multivariate data analysis, and sensory methods (GC-olfactometry and a sensory panel). Both investigated materials (PE and PP) showed similar degradation products (aldehydes, ketones, carboxylic acids, alcohols, and lactones) which increased dramatically with increasing aging time and the lack of stabilization. Oxidation products, mainly carbonyl compounds, were responsible for the odor of the investigated materials. The main odor drivers were unsaturated ketones and aldehydes with a chain length between six and nine C-atoms. Interestingly, similar odor patterns were found for both stabilized and unstabilized samples, indicating that similar formation processes take place independent of the stabilization.